1. Field of the Invention
The invention relates to forming a paper web on a paper machine. More particularly, the invention relates to testing apparatus designed to measure drainage of water from a pulp furnish on that portion of the paper machine which forms the paper web or sheet.
2. Description of the Prior Art
In forming a pulp or paper web, a pulp furnish comprising a dilute slurry of fibers is typically flowed onto a moving wire. The wire travels across a series of foils, suction boxes and rolls which apply vacuum forces to dewater or drain away the slurry water while retaining the fibers on the wire and eventually forming the paper or pulp sheet. The formed sheet then proceeds to the paper machine pressing and drying sections for further water removal.
It is well known that paper web strength increases as the web becomes drier. The amount of water removed during forming determines the strength of the web. If drainage is slow, the machine speed may have to be reduced because the web may not be strong enough to survive the stresses which are exerted on the sheet in leaving the forming section and undergoing pressing operations. Conversely, a small improvement in the rate of drainage of water from the sheet on a wire limited machine, i.e., where wire speed is a production bottleneck, will translate into increased production. An increase in the total amount of water removal before the paper web enters the dryer section may mean increased profitability through lower drying costs. A 1% decrease in moisture content of the wet web means a 4-5% decrease in required dryer section capacity.
The driving forces for water drainage from the furnish slurry on the forming wire of the paper machine have been suggested to include: (1) hydrostatic pressure from the weight of the fiber water slurry on the wire; (2) inertial pressure from the angular impingement of the stock jet from the head box slice onto the wire; (3) hydrodynamic vacuum forces generated by motion of the wire over table rolls and drainage foils; (4) externally generated vacuum forces applied by suction boxes under the wire; and (5) pressure from rolls such as lump breaker rolls and couch presses. Drainage on a paper machine depends primarily upon vacuum forces employed during forming and the rate at which water can be removed from a particular furnish. The fiber composition of a furnish, furnish refining schedule, the total non-fiber additives package as well as the physical layout of a machine each play major roles in achieving a particular drainage level.
Improving paper web forming efficiency on a paper machine is a continual goal of all papermakers. In determining performance of a furnish on the paper machine, it is necessary to know how fast water drains from a particular furnish, and how much water can be expected to be removed by vacuum on the forming wire. Papermakers rely on drainage measurements in one form or another as a convenient rule of thumb to predict runnability of a given papermaking furnish on the former of a paper machine. Most present drainage testers such as Canadian Standard Freeness (CSF), Schopper-Riegler, Williams Precision, Drainac, etc. measure principally the rate at which water drains from a furnish before the mat is formed. Drainac is a registered trademark of Bolton-Emerson, Inc., Lawrence, Mass. The result of such tests is often called the "freeness" of a pulp furnish. The contribution of dewatering through a mat is generally ignored, however, resulting in freeness values that do not relate to performance on the paper machine. Freeness drainage testers presently available do not give a complete description of behavior of a furnish as it is formed into the paper web on the machine. Changes in furnish, refining pattern and use of chemical additives will change relationships between freeness and runnability even though the freeness value itself may not change or change in a way that is unrelated to runnability.
Increasing refining of a pulp furnish is a method of increasing the strength of the paper formed on the paper machine. The greater the refining energy imparted to a pulp, the greater is the strength potential of the web formed on the machine. However, the impact of increased refining generally is to slow the drainage rate on the wire. With chemical pulps, as degree of refining increases, there is an increase in time required for the pulp to drain and less water can be removed. However, papermakers know that furnishes containing mechanical pulps drain faster on a paper machine than do chemical pulps, even when refining levels are equal. For example, a groundwood furnish having a Canadian Standard Freeness of 150-200 ml will dewater on a fourdrinier as fast as a kraft furnish having a CSF of about 500 ml.
Freeness measures are still used, however, because no other tools are available. With experience, the papermaker has been able to set up "correction factors" that appear to work for some purposes. However, for many purposes the impact on drainage resulting from operating changes is unpredictable and present methods require on-machine trials. As an example, wet end retention aids and the like tend to decrease drainage sometimes so significantly that production must be reduced. There is, therefore, a need in industry to measure drainage in a way that responds to furnish variations, refining levels and the like and produces a result that directly relates to actual machine performance.